20w-50 and straight 50

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CCI

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Without getting into the why, just from a technical perspective, what happens when you mix straight mineral SAE 50 with 20w-50? Mayne 2 qts of each? Or 3 qts SAE 50 with 1 qt 20w-50?
 
Good to see you here MT, I was hoping you'd respond. OK, now the reason why -- this is a 40 year old motor that has an honest running compression check of 215 pounds on one cylinder and a little over 230 on the other. Stock head bolts didn't cut it, had to go to aftermarket bolts to stop blowing head gaskets and vaporizing aluminum mating surface. It is kickstart only, it will break the back tire loose on dry pavement while already rolling without touching the clutch, and when it is hot it bleeds 20w-50 from everywhere but it only leaks about a quart every 750 to 1,000 miles on straight 60. I am OK with that, I have to be, there is no practical way to fix the problem. But in the winter, well, kick starting that bike below about 50 degrees is just comical, below 40 degrees it's not even worth trying. I'm going to do a little mixology and will report back. Thanks!
 
How about some Amsoil SAE60? It's supposed to behave like a 20w or 25w in cold-but-worth-riding temps. Or so dealers say.. You could use the same oil all year, but I'm sure it's expensive.
 
Originally Posted By: CCI
Without getting into the why, just from a technical perspective, what happens when you mix straight mineral SAE 50 with 20w-50? Mayne 2 qts of each? Or 3 qts SAE 50 with 1 qt 20w-50?
I do that with straight 40 and 15/40, but I never see sub-zero temperatures. I can see some ways it might be better (less shear and ring gum mostly) but I can't see why it'd be better for you than 20w50 if your concern is leakage, unless you're only hoping to reduce leakage while its warming up. Isn't the effective viscosity at operating temperature going to be the same?
 
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Isn't the effective viscosity at operating temperature going to be the same? I've always thought so, but it seems there must be more to the story. 20w-50 turns very thin and comes out all over at 100 degrees ambient, while the SAE 60 stays in rather well except for the bigger holes. The motor sounds awful at running temp with 20w-50, sounds good with 60. Or 50 in colder weather. I have no idea.
 
Originally Posted By: CCI
Isn't the effective viscosity at operating temperature going to be the same? I've always thought so, but it seems there must be more to the story. 20w-50 turns very thin and comes out all over at 100 degrees ambient, while the SAE 60 stays in rather well except for the bigger holes. The motor sounds awful at running temp with 20w-50, sounds good with 60. Or 50 in colder weather. I have no idea.
I'm not completely clear on this but I THINK VII's can show temporary shear-induced loss of viscosity, which isn't shown by straight weight oil, so maybe that's part of the difference.
 
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Originally Posted By: CCI
20w-50 turns very thin and comes out all over at 100 degrees ambient, while the SAE 60 stays in rather well except for the bigger holes. The motor sounds awful at running temp with 20w-50, sounds good with 60.
Redline makes 20W60.
 
Just re-read your post, I don't think I gave a very good answer last time. Three concerns: Foremost is that I cannot kick this bike in cold weather with straight 50 or 60. Second is the multi-viscosity oils just don't hold up. I don't know why, but it just turns thin in no time and the motor sounds terrible. You can feel the difference at over 80 degrees ambient. The motor is old, everything that has not been rebuilt is worn out, to the point where I can't run hydraulic lifters anymore because they collapse at idle. Oil pump hasn't produced more than 12 psi hot in decades, probably about 3 psi at an idle. Third is leakage, oddly enough the thicker oils leak worse when the motor is cold (more pressure I suppose), but once it is hot, the multi-vis oils come out faster. So today I drained out the SAE60 that had about a thousand miles on it and looked awful and put in 2 qts 20w-50 and 2 qts SAE50. The motor sounds much better on this, we'll see what a little time and a few miles do. As for effective viscosity at temperature, I know it's supposed to work that way, but over about 80 degrees ambient on any multi-vis oil the motor sounds like somebody threw the entire middle school marching band down the stairs. On straight 60 it sounds great. I have no idea why.
 
Originally Posted By: CCI
Just re-read your post, I don't think I gave a very good answer last time. Three concerns: Foremost is that I cannot kick this bike in cold weather with straight 50 or 60. Second is the multi-viscosity oils just don't hold up. I don't know why, but it just turns thin in no time and the motor sounds terrible. You can feel the difference at over 80 degrees ambient. The motor is old, everything that has not been rebuilt is worn out, to the point where I can't run hydraulic lifters anymore because they collapse at idle. Oil pump hasn't produced more than 12 psi hot in decades, probably about 3 psi at an idle. Third is leakage, oddly enough the thicker oils leak worse when the motor is cold (more pressure I suppose), but once it is hot, the multi-vis oils come out faster. So today I drained out the SAE60 that had about a thousand miles on it and looked awful and put in 2 qts 20w-50 and 2 qts SAE50. The motor sounds much better on this, we'll see what a little time and a few miles do. As for effective viscosity at temperature, I know it's supposed to work that way, but over about 80 degrees ambient on any multi-vis oil the motor sounds like somebody threw the entire middle school marching band down the stairs. On straight 60 it sounds great. I have no idea why.
Yeh. I believe straight weights have a lot going for them. You MIGHT find a mix of 60 and 20W50 will be better and still allow you to kick start. You could perhaps gradually evolve in that direction by topping up with straight 60 (I'm assuming you'll still have enough leakage to require fairly frequent topups) and seeing how it goes. I ended up with my mix by topping up Delvac MX 15W40 with local SAE40, then getting a massive rocker cover leak. I probably could, and perhaps should, just run the straight 40 but I'm not sure of its quality used alone.
 
Originally Posted By: Ducked
Originally Posted By: CCI
Isn't the effective viscosity at operating temperature going to be the same? I've always thought so, but it seems there must be more to the story. 20w-50 turns very thin and comes out all over at 100 degrees ambient, while the SAE 60 stays in rather well except for the bigger holes. The motor sounds awful at running temp with 20w-50, sounds good with 60. Or 50 in colder weather. I have no idea.
I'm not completely clear on this but I THINK VII's can show temporary shear-induced loss of viscosity, which isn't shown by straight weight oil, so maybe that's part of the difference.
Basic description of shear-thinning https://www.oronite.com/paratone/shearloss.aspx Includes the line "Note that larger coils are subject to larger forces when the oil is exposed to shearing stresses.... so therefore, high molecular weight molecules are more vulnerable to permanent viscosity loss." I can't offhand see why this wouldn't apply to temporary viscosity loss as well, though apparently there isn't a consistent relationship between the two. http://papers.sae.org/801390/ That aside, since youre operating at the upper end of the viscosity range, you might expect to see larger viscosity losses.
 
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